EconomicGeology
Vol. 85, 1990, pp. 1738-1758
Evolution of the Monte Negro Acid SulfateAu-Ag Deposit, PuebloViejo,
DominicanRepublic:Important Factorsin Grade Development
JOHNL. MUNTEAN,*STEPHEN
E. KESLER,
Departmentof GeologicalSciences,Universityof Michigan,Ann Arbor, Michigan48109
NORMANRUSSELL,AND JOSEPOLANCO
RosarioDominicana,S.A.,P.O. Box 944, SantoDomingo,DominicanRepublic
Abstract
Thisstudywasundertakento determinethe geologicfactorsthat were importantin making
PuebloViejo, whichcontainsover 600,000 kg of goldin relativelyhigh-gradeore, one of the
few acid sulfatedepositsto be mined economicallyon a large scale.Orebodiesat Pueblo
Viejo, includingMonte Negro, the focusof this study,are found arounda maar-diatreme
complexthat formedin the upper part of the Lower CretaceousLosRanchosFormation.Ore
is hostedby spilite,spilite-derivedconglomerate,andintramaarcarbonaceous
sandstone
and
mudstone.Regionalseawateralterationof theserockswasoverprintedby two stagesof advancedargillicalteration,both of whichare associated
with preciousmetals.StageI produced
deep alunite + quartz + pyrite and overlyingkaolinite+ quartz + pyrite assemblages
and
depositedgoldin association
with disseminated
pyrite in wall rock.StageII, whichoverprinted
stageI, produceddeep pyrophyllite ___
diasporeand an overlying(recrystallized)silicacap.
Pyrite _ sphalerite_ enargiteveinsthat probablyformedin responseto hydrofracturingof
the silicacap containgold gradesof asmuchas 100 ppm. Vein densityand assaydatashow
that approximately60 percentof the goldat MonteNegrois in disseminated
stageI ore, with
the remainderin stageII veins.
Phaseequilibriaand sulfurisotopethermometryshowthat stageI alterationtook place
below 260øC at pH valuesbetween 2 and 3. StageII pyrophyllite___
diasporeformation
occurredabove285øC, froma fluidwith aninitial pH of 1 to 1.5, whilethe silicacapsformed
below 220øC. Geologicconstraintsindicatethat stageI wall-rockgold at Monte Negro was
depositedby sulfidationof Fe-rich wall rock, whereasstageII vein-hostedgold could have
formed by cooling,boiling, or mixing with overlyinggroundwater. Use of the programs
SOLVEQ and CHILLER to evaluatethe relativeefficienciesof theseore-formingprocesses
confirmsthat sulfidationwascapableof depositingdisseminated
stageI gold,whereasstage
II vein goldwasprobablydepositedby boilingrelatedto pressurereleaseduringhydrofracturing and subsequentmixingwith groundwater.
These observations
indicatethat developmentof grade and tonnageat PuebloViejo was
greatly facilitatedby (1) the porous-permeable
maar-diatremecomplex,which hostedlarge,
shallowhydrothermalcells and probablyservedas a conduitfor risingmagmaticgases,(2)
the presenceof iron-richwall rock, whichcausedearly depositionof "background"disseminated gold by sulfidation,and (3) later hydrothermalactivity which formedimpermeable
silicacapsthathydrofractured
to createthe high-gradeveinoverprint.Remobilization
of stage
I backgroundgoldinto stageII veinsmightalsohavebeen an importantfactorin upgrading
the deposits.Explorationfor similardepositsshouldfocuson areasof explosivevolcanismin
andesitic island arc terranes.
Introduction
ALTHOUGHacid sulfatepreciousmetal depositsare
widespread(Healdet al., 1987), surprisingly
few have
the gradeand tonnagerequiredfor large-scalebulk
mining.Manydeposits,
includingGoldfield(Ransome,
1909) and E1 Indio (Siddeleyand Araneda, 1986),
havebeenminedby selectivemethods,andin others,
suchasSummitville,wherebulkminingmethodshave
* Presentaddress:SantaFe PacificMining, Inc., 250 S. Rock
Blvd., Suite 100, Reno, Nevada 89502.
0361-0128/90/1150/1738-21
$3.00
been applied, resultshave been discouraging.
The
most prominent exceptionto this rule, the Pueblo
Viejo district in the DominicanRepublic, has producedover 50 metric tonsof oxideore gradingapproximately4 ppm, all of which hasbeen extracted
by open-pit bulk mining methodsat a rate of about
10,000 metric tonsper day. Approximately100 million metric tonsof slightlylower gradesulfideore,
which is alsoamenableto bulk mining,underliesthe
oxideore in the depositsat PuebloViejo. This report,
whichfocuseson the Monte Negro deposit,the sec-
1738
MONTENEGROAu-AgDEPOSIT,
DOMINICAN
REPUBLIC
1739
ondlargestorebodyat PuebloViejo, wasundertaken pilli tuff,andcarbonaceous
lacustrine
sediments
(Figs.
to identifythe geologicfactorsthatpermittedPueblo 1 and 2).
Viejo to be an economicsuccess.
Primarymagmaticmineralsin the Los Ranchos
Formationhavebeenalmostentirelyalteredto albite,
GeologicSettingof PuebloViejo Mineralization
quartz,cakeitc,
ohiorite,illitc, smectite,
epidote,ac-
tinolite,andprehnite.Evidencethat theseminerals
PuebloViejoisin the upperpartof theLowerCre- wereproducedby seawateralterationratherthanas
taceousLos RanchosFormation, a series of volcanic a propylitichalorelatedto PuebloViejo mineralizaand volcaniclastic rocks on the east side of the Cor-
tion is indicatedby strontiumisotopecompositions
dilleraCentral,theaxialmountain
rangeofHispaniola of calcitein amygdules,whichare similarto Creta(Bowin, 1966). The Los RanchosFormationconsists
of a lowercomplexof pillowedspiliteflowsandkeratophyreandquartzkeratophyreflows,tuffs,andintrusions,which are overlain by volcaniclasticsedimentsandlahars.Theseare overlain,in turn, by an
extensivespiliteunit that lackspillowsandcontains
lensesof sedimentwith plant fossils(Kesleret al.,
1991). A maar-diatremecomplexin the upper part
of the LosRanchos
Formation,whichformedduring
the latestphaseof volcanism,hoststhe PuebloViejo
ceousseawater(Kesleret al., 1991).
The PuebloViejo districtcomprises
severalacid
sulfateorebodies,of which Moore and Monte Negro
are the two.largest(Fig. 2). The depositsare found
at or nearthe marginof the maar-diatreme
complex,
wheretheyformmushroom-shaped
bodiesthatoverprint the regionalseawateralteration.MonteNegro,
whichis foundalongthewesternmarginof themaar-
diatremecomplex
contains
14 millionmetrictonsof
oxideoregrading3.35 ppmgoldand7.6 ppmsilver
district (Sillitoeand Bonham,1984; Russelland Kes- (0.8 ppmgoldcutoff),whichoverlies
a 37-millionsulfidereserve,grading3.95 ppmgoldand
ler, 1991).The complexincludesfragmented
spilite, metric-ton
et al.,
redeposited
pyroelastic
rocks,quartzeye-bearingla- 22.4 ppmsilver(2.5 ppmgoldcutoff)(Russell
70 ø 15'
•!tlanti•
Ocean
k..f
Caribbean
70 ø lO'
19'00'
: + 19ø00'
--
Viejo
•<•<_-v
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LasLagunas
Formation
HatilloFormation
LOS
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UPPER
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EXPLANATION
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CRETACEOUS
RANCHOS
FORMATION
PuebloViejoSedimentary
Rocks (Maar)
•[-• PlatanalSpilite
•
Undivided
Keratophyre
and
70 ø 15'
19055
Quartz Keratophyre
Maimon Formation (Undatedl
Thrust
Fault
Fault
0
i
2
3
I
I
I
I
4 Km
I
Bedding
FIG. 1. Geologicmapof the centralLosRanchosFormation(after Kesleret al., 1990).
N
1740
MUNTEAN,KESLER,
RUSSELL,
AND POLANCO
v
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EXPLANATION
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CRETACEOUS
HATILLO
FORMATION
•
.•
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Diorite
UPPER
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Limestone
Conglomerate.
mudstone
LOWER
CRETACEOUS
LOS RANCHOS
FORMATION
•
SpiliteWacke
•F•-• LapilliTuff
•-•
Carbonaceous
Mudstone
500
1
•--• Carbonaceous
Sandstone
Ore
i'•
SpiliteDerived
Conglomerate
Fault
•
•
Spilite
Volcaniclastic
Rocks
Fold
1000m
I
Zone
Bedding
Cross-Section
(Fig.4)
FI(::;.
2. Geologicmapof the maar-diatreme
complexat PuebloViejo showinglocations
of the Monte
Negro and Moore orebodies(after Russellet al., 1990).
1981). The Monte Negroorebodyisrestrictedin volume in the spilite but spreadsout in the overlying
intramaarsedimentaryrocks.The orebodyis largely
undeformedandiscut by postore,Eocene(?)diorite
dikes. Although alunite from the Moore orebody
plexlyzonedbothspatiallyandtemporallyhavebeen
delineatedat Monte Negro. StageI is characterized
bythecomplete
replacement
oftheregionalseawater
alterationby a zonedsequence
of advancedargillic
alterationconsisting
of deep alunite-quartz-pyrite-
yieldsa 66-Ma 4øAr/39Ar
age (Kesleret al., 1981), rutfie, which underlies a more extensive kaolinitegeologicandfossilevidencestronglypointsto a Lower quartz-pyrite-rutilezone.Disseminatedgoldis assoCretaceousage for the mineralization(Russelland ciated with wall-rockpyrite and is believedto have
Kesler, 1991).
beendeposited
duringformationofthekaolinitczone.
Stage
II
advanced
argillic alteration,which is charZoningand Paragenesis
of AdvancedArgillie
Alteration
Introduction
and Precious Metal Mineralization
acterizedby widespread
pyrophyllite_ diaspore
and
high-level caps of massivesilicification,has over-
printedandreplacedstageI alterationto varyingdeTwo stagesof advancedargillicalterationand as- grees. High-gradegold-bearingsulfideveins were
sociatedpreciousmetal mineralizationthat are com- emplacedduringstageII, probablyin response
to hy-
MONTENEGROAu-AgDEPOSIT,DOMINICANREPUBLIC
Unreplaced
1741
/
Partiall'•
/
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Replaced
......
Completely
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Replaced / //
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II --
•'
TIME
FIG. 3. Parageneticdiagramshowingtemporaland depth zoningof mineral assemblages.
Depths
are approximate.Positiveslopesindicateprogradeassemblages,
while negativeslopesindicateretrograde
assemblages.
APS = alumino-phosphate-sulfate
minerals,Dia -- diaspore,Kao -- kaolinite, Pyroph
-- pyrophyllite,Qtz -- quartz,Silica= recrystallizedmetastablesilicaphases.
drofracturingof the high-levelsilicacaps.Figure 3,
which is a parageneticdiagram,Figure 4, which is a
seriesof crosssectionsshowinggeology,alteration,
gradedistribution,and Figure 5, which is a seriesof
photomicrographs
showingkey parageneticrelationships,attemptto illustratethis spatialand temporal
rangeof Na/(Na + K) moleratios(0.2-0.8, Table 1)
bothin the wallrockandveinlets.Thesecompositions
suggestsolid solutionbetween alunite and natroalunite, despiteexperimentaldata (Stoffregen,1988)
permitting an assymetricsolvusbelow 350øC. The
alunite showsno spatialor temporalcompositional
zonation of alteration and mineralization and should
zoning,andthe observedrangeof compositions
could
be referredto asdetaileddescriptions
of the alteration reflect multiple pulsesof fluid of slightly different
and mineralizationpresentedbelow.
compositions
duringthe formationof the alunitezone
or, more likely, retrograde reaction of alunite with
later fluids.
StageI alteration and mineralization
Aluminophosphate-sulfateminerals, including
In spiliteandspilite-derivedconglomerate
the ka- svanbergiteandwoodhouseite,
are locatedalongthe
olinite-quartz-pyrite-rutilezone containspseudo- periphery of the alunitezone.Thesemineralsare not
morphicpatchesof kaolinitein what were oncefeld- directlyassociated
with either aluniteor kaoliniteand
sparphenocrystsites.Other phenocrystsiteswhich occur with quartz as discrete euhedral to subhedral
were probablymaficmineralscontainquartz,pyrite, grains(<50 •m) in the wall rock or more rarely as
andrutile.The alteredgroundmass
consists
of cloudy, veinletswith no alterationenvelopes.Althoughthey
fine-grained,
anhedralquartzwith disseminated
py- couldbe partlythe resultofapatitereplacement(Stofrite and rutile. In the lacustrine sandstone and mudfregenandAlpers,1987), localintroductionof phosstone, kaolinite is much less common and occurs as phorousis neededto accountfor zonescontainingas
irregularmasses,and sideriteis replacedby pyrite.
much as5 percent of theseminerals.
Inclusionsof gold-richelectrumand tellurides,as
Thereisa sharptransition,
overa fewmeters(Fig.
4B), from kaolinite-bearing
rocksto an underlying well asCu-As-Pb-Agsulfides,havebeen recognized
zone containingphenocrystsitesthat are occupied in pyrite framboidsand euhedra in the wall rock in
by clearquartzandalunitein a groundmass
of fine- the kaolinite zone and are believed to have formed
grained(<50 •m) anhedralquartz,alunite,pyrite, during stageI sulfidationof wall rock in the kaolinite
andrutile.Thisalunitezone,whichis entirelyin spi- zone (Kettler, 1989).
lite, hostsveinletsof quartz,alunite,andpyritethat
Retrogradingof stageI alteration, which is indiexhibitmutualequilibriumtextures(Fig.5A) andap- catedby localreplacementof aluniteby sericite,sugpear to be contemporaneous
with the wall-rock al- geststhat the stageI thermalevent decayedbefore
teration. Microprobe analysesof the alunite showa the beginningof stageII alterationandmineralization.
1742
MUNTEAN, KESLER, RUSSELL,AND POLANCO
N A DDHDDHDDHDDH
B
o
400 -I
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!
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FIG.4. Crosssections
throughMonteNegroalongsection
•-
lineA-B-C
inFigure
2.Control
isprovided
bydiamond
drillcore
200
•
100
•F_J•E
LEVEL
•(.
APS
=alumino-phosphate-sulfa
minerals.
C.
Stage
IIAdditional
alteration
D. Goldgradedistribution.
E. Silvergradedistribution.
/ •••j/
l
holes,
shown
in
the
cross
section
A.
Geolo
B.
Stag
Ialte
controlfor goldandsilvergradesisprovidedby reversecirculation
5
.
t_ _1•-lUppm
drill
holes
not
shown
inthe
section.
J
(Fig. 4C). Petrographicevidence,discussedbelow,
suggests
that stageII is a distinct,later eventthat has
StageII alterationis characterized
by widespread overprintedandreplacedstageI alterationto varying
pyrophyllite_ diaspore,high-levelcapsof massive degrees.In the southernend of Monte Negro (Fig.
silicification,
andassociated
gold-bearing
sulfideveins 4C), the stageII alterationwassointensethat noves-
StageH alterationand mineralization
MONTENEGROAu-AgDEPOSIT,
DOMINICANREPUBLIC
1743
tigesof stageI alterationremain,and it is possible not observedtogether or with pyrite. The massive
that somestageII alterationextendedbeyondstage silica is characterizedin thin sectionby numerous
I alteration.
overlappingveinletsconsisting
soleyof clearquartz,
Where stageII alterationoverprintedthe alunite, which cut the stage I quartz-pyrite-rutile matrix.
zoneof stageI, diaspore
replacedalunitelocally,and These veinletscoalesceat kaolinitc patches,where
pyrophylliteformed veinletsand patchesthat cut coarse,clear quartzhasreplacedkaolinitc(Fig. 5E).
alunite,quartz,and pyrite. Pyrophyllitecontentof
Many textures in the silicified rocksresembleretherelictalunitezonereaches
approximately
20 per- crystallizedsilica,suchaschalcedony,opal,or amorcent (by volume),but is usuallymuchless.The fact phoussilica.Clearcrosscutting
quartzveinletswhich
thatpyrophyllitealsoreplacedstageI retrograde
ser- are filled with fine-grained,anhedralquartz,showno
icite locally indicatesthat stageII alterationwas a developmentof crystalswith the c axisperpendicular
distinct,later eventafterthe collapseof the stageI to the veinletwall andare interpretedto haveformed
system.
by recrystallizationof amorphoussilica (Fournier,
IncipientstageII overprintingof the kaolinitczone 1985). Boundaries
betweencloudyandclearquartz,
is characterizedby monomineralicpyrophyillite as observedunder uncrossedpolars,are transected
veinletsthat cut the quartz-richmatrix and coalesce by boundariesbetweenquartzgrainsof uniformexat kaolinitcpatches,wherepyrophyllitehasreplaced tinctionwhenviewedunder crossedpolars(Fig. 5F
kaolinitcto varyingdegrees(Fig. 5B).Furtherstage and G). Plumosequartz, which may alsobe recrysII overprintingresultedin the completereplacement tallized metastablesilica (Sanderand Black, 1988),
of kaolinitcandextensive
replacement
of quartzby hasbeen observedlocallyin the silicifiedrocks.
pyrophyillite(Fig. 5C), leavingsomezonesin which
In the intenselysilicifiedcaps,the rock is cut by
pyrite andrutile are the only remainingstageI min- sharplywalledveinletsof pyrophyllite.Kaolinitcand
erals.Little additionalwall-rockpyrite appearsto minoramountsof silicahavebeenreplacedpartlyby
havebeendeposited
duringformation
of stageII pyrophyllite, and the pyrite exhibitssmoothgrain
boundaries
with pyrophyllite,suggesting
equilibrium
betweenthe two mineralsduringstageII.
pyrophyllitein the silicifiedrocks.Abovethe silicified
zones,pyrophyllitehasreplacedboth kaolinitc and
quartz extensively,much like the zone of massive
pyrophyllitebelow the silicifiedcaps.The amountof
Diasporeis presentlocallyin deeperpartsof the introducedstageII silica and the pervasiveness
of
relict kaolinitczonethat haveundergoneextensive pyrophylliteformationvary inversely.
replacementby pyrophyllite.It formspatchesthat
Gold-bearingsulfideveinscut stageI and are inappearto be in equilibriumwith pyrophyllite(Fig. terpretedto be contemporaneous
with stageII alter5D) in someplacesandtobereplaced
bypyrophyllite ation. The veins are commonlylessthan 2 cm thick
in others.In deep partsof the relict kaolinitczone, and have sharpwalls in the fine-grainedsediments
replacementof stage I aluminophosphate-sulfate
and more irregular boundariesin the conglomerate
mineralsby diasporehasalsobeen observed.
and spilite. Alteration selvagesare uncommon,alMassivesilicacapsare foundin the relict kaolinitc thoughcm-scaleenvelopesof silicificationand,more
zoneaboveareasof massive
pyrophyllite___
diaspore. rarely,pyrophylliteformationcanbe seenlocally,and
They are mostintensein the conglomeratic
horizons swarmsof veinsare closelyassociated
with areasof
thatdirectlyoverliethe spilitewheretherockconsists intensesilicification.
Severalepisodesof veiningare
almostentirelyof microcrystalline
quartz,similarto indicatedby crosscuttingveins of severaldifferent
jasperoid.Pyrite,whichiscoarserthanthe finelydis- orientations,suggestingvein emplacementresulted
seminated
pyritedescribedabovefor stageI, isabun- from hydrofracturing.
dantin the silicifiedrocks,althoughit is difficultto
The veinsare distinctivein being rich in sulfides
determine
whetherit wasdeposited
alongwithsilica. and poor in gangueminerals.Pyrite, by far the most
Minor enargiteandsphaleritearealsopresentbut are commonmineralin the veins,formsfine-grained,biT•,BLE1.
Na/(Na+K)
Microprobe Determination of Na/(Na+K) Mole Ratiosin Alunite
I
2
3
4
5
6•
7•
0.649
0.417
0.399
0.792
0.795
0.205
0.197
Sampledescriptions:
1, 2, 3 = DDH195, 246.9 m, wall-rockalunitedirectlynextto pyrite-alunite-quartz
veinlet;4 -- DDH195,
236.5 m, wall-rockalunitein phenocrystsite;5, 6, 7 -- DDH195, 246.9 m, alunitefrom pyrite-alunite-quartzveinlet
• Analyses
6 and7 were20 •m apartona singlegrain;otheranalyses
wereonseparate
grains
The alunitewasanalyzedwith the Camecaelectronmicroprobeat the Universityof Michiganusingan accelerating
voltageof 10
kV; naturalstandards
were used,includingMarysvalealunite(S, A1,K), jadeitc (Na), barite (Ba),hornblende(Fe), celestite(Sr),and
apatite (P); a diffusebeam (10-29 •m in diam) wasusedto minimize vaporization
1744
MUNTEAN, KESLER, RUSSELL, AND POLANCO
O.05mm
.j '.
PY
,
0.25mm
0.25mm
FIG. 5.
laterally symmetrical, colloform layers indicating
open-spacefilling. The central parts of many veins
are filled by enargite and sphalerite,with enargite
morecommonin the centralpart of the orebodyand
sphaleritemore commonin peripheral parts of the
deposit.Bothenargiteandsphaleritecanformmonomineralicveinsandmasses
locally.Sphaleriteis com-
monlycolorbanded,although,microprobeanalyses
indicateno changein Fe, Mn, or Cd with color.The
Fe contentof the sphaleriteislow, with molepercent
FeS rangingfrom nondetectablelevelsto 0.38 percent. Other; lessabundant, sulfide-sulfosaltvein min-
eralsincludeZn-bearingtennantite(actuallytennantite-tetrahedritewith As/Sbmoleratiosrangingfrom
MONTE
NEGRO
Au-Ag
DEPOSIT,
DOMINICAN
REPUBLIC
1745
Au-Te
' 0.25mm
O.01mm
P
"Qz
0.25mm
FIG. 5. Photomicrographs
illustratingparagenetic
relationships.
A. Back-scattered
electronimage
of samplefromalunitezoneshowing
an equilibriumintergrowthbetweenalunite,pyrite,andquartz
in a veinlet.B. Pyrophy!!ite(light, fine-grainedmaterial)crosscutting
and replacingkaolinitcpatch
(darkmaterial).Pyrophylliteis alsocrosscutting
quartzin discreteveinlets.Crossedpolars.C. Pyrophy!!ite(light,fine-grained
material)replacingquartz.Crossed
polars.D. Euhedralcrystals
ofdiaspore
andpyrophyllite(light,fine-grained
material).Crossed
polars.E. StageII sfiicification
(uncrossed
polars)
showingveinletsof clearsilicacrosscutting
cloudyquartzandreplacingkaolinitcin phenocryst
sites.
F. StageII silicification.
Uncrossed
polars.Note the boundaries
betweenthe clearandcloudyquartz.
G. Samefield of view asF, but under crossedpolars.The blackoutlinemarksthe boundariesof the
phenocrystsite occupiedby clear silicain F. H. Backscattered
electronimageof calaveritewith Zn
tennantitefillingsitebetweentwopyritegrainsandfillingmicrofracture
in a pyritegrain.I. Pyrophy!!ite
(light material)filling cracksbetweensphaleriteand crosscutting
microcrystal!inc
quartz. (Crossed
polars).Abbreviations:A! = (natro)alunite,Au-Te = calaverite,Di = diaspore,Ka = kaolinitc,
P = pyrophy!lite,Py = pyrite, Qz = quartz, Sp = sphalerite,Ten = Zn tennantite.
3.1-9.6), Pb-bearing sulfosalts(with compositions
closeto As-bearingbournonite,zinckenite,andboulangerite),stibnite,andgalena,which occureither as
inclusionsor asrims or fracturefillingsin pyrite, enargite, or sphalerite.
sions,lessthan 10/•m in diameter,in pyrite. Although
not yet observedat Monte Negro,Au-rich electrum
alongpyritegrowthzonesin sulfideveinsisofprimary
importance at the Moore orebody (Kesler et al.,
1985). Silver-bearingphasesobservedin the veins
Gold is found in the veins as calaverite with minor
includeZn-bearingtennantite(0.75-1.73 wt % Ag),
silver and copper (•5 wt %), which occursas blebs enargite(0.02-0.13% Ag), andPb-bearingsulfosalts
in Zn-bearingtennantitewhich fills fracturesin vein (up to 0.53% Ag).
Interstices between sulfides and the centers of some
pyrite (Fig. 5H) or intersticesbetweenpyrite grains.
Somecalaveriteis associated
with a mineralhavinga veins containfeathery, plumosequartz and/or excompositioncloseto As-bearingbournoniteasblebs tremelyfine grained,milky chert,both of whichare
in tennantite. Calaverite also occurs as discrete incluprobably recrystallized metastablesilica. Barite is
1746
MUNTEAN,
KESLER,
RUSSELL,
ANDPOLANCO
presentlocallyin theveincentersor asrimsonpyrite rocks,near the neckof the system,althoughhigh Au
grains.Pyrophyllite,which is often presentin the andAg gradesandsignificantdeepAg mineralization
veins,appearsto have postdatedsulfidemineraliza- in spilite in the southernpart of the section(DDH
tion,whereit fillscracksbetweensulfidegrainsand 235) appearto be separatefromthe mainmushroomhasreplacedtheveinsilica(Fig.5I). Thepresence
of shapedorebody.The crosssectionsalsoshowthat Au
thispyrophyllite
shows
clearlythatadvanced
argillic and Ag mineralization is restricted to areas of adalterationpersistedthroughvein emplacement
and vancedargillicalteration;areasexhibitingonly presulfide formation and indicates that the veins and ad-
stageI seawateralteration are barren.
vancedargillicalterationresultedfrom the samefluid.
The possibilitythat sulfideveinscarriedthe bulk
of the Au at Monte Negro (Kesleret al. 1981) was
testedwith vein assays
usedin conjunctionwith bulk
Distributionof preciousmetals
Ore andselectedrockassays
were usedto obtain
additionalinformation
on the mineralogy,
distribution,andparagenesis
of Au andAg at MonteNegro,
andto determinehowpreciousmetalgradesrelate
to stages
andtypesof alterationat MonteNegro.Telluriumandgoldassays
fromdiamond
drill core(Fig.
6A) showa positivecorrelation.Most assays
plot
abovethe line representing
the Te/Au massratio of
calaverite,indicatingthepresenceof excess
Te. Trace
Te detectedin enargitefromMonteNegro(upto 0.03'
assaysand measurementsof vein densitiesfrom four
core holes.The bulk intervalgrade(IG) canbe calculatedfromthe vein density(VD), veingrade(VG),
andwall-rockgrade(WG) for a givenintervalby the
equation:
IG = (5/3)(VD)(VG)+ (1 - (5/3)(VD))(WG) (1)
(where 5/3 convertsvein densityfrom a volumeper-
centto a weightpercent,assuming
a 5-g/cm3density
wt %) isnotabundantenoughto accountfor theexcess for sulfide vein material, since >90% of the veins
contained>90% pyrite, and 3 g/cm3 for wall rock,
basedon densitymeasurements
discussed
below).If
the bulk interval grade (IG) is measuredindependently,eq (1) canbe rearrangedandusedto estimate
ler et al., 1981). The positivecorrelationbetweenAu wall-rockgradeandthe percentageof goldandsilver
and Ag (Fig. 6B) suggeststhat electrum also hosts contributedby the veins.
preciousmetalsat MonteNegro,despitethe scarcity
The results(Table2) showthat, on average,stage
of observations of electrum.
II gold-bearingsulfideveinscontributed40 percent
Geologiccrosssections
with preciousmetalgrade of the Au and 47 percentof the Ag in the Monte
distributions
(Fig.4D-E) showthatAu andAg min- Negro ore, indicatingthat there is significantwalleralizationis relativelyrestrictedin the spiliteand rock Au and Ag. The averagemeasuredvein grade
spreadsout laterallyin the overlyingsedimentaryfor the fourholesusedin thistestis21.1 ppmAu and
rocks.Mosthigh gradesoccurin the sedimentary 160.8 ppm Ag, yielding an Ag/Au ratio of 7.6. The
Te. Although
notyetdetected
atMonteNegro,native
tellurium,hessitc(Ag2Te),Au-Agtellurides(with
compositions
closeto krennerite),andcoppertellurideshavebeenobserved
attheMooreorebody(Kes-
200
40O
R=0.47
18o A
316DATA
R =0.66
350 B
43O DATA
160
300
ß
•' P50
ß
100
-
'"'
mlm.•m•m•
0
0
2
mmm_m
,
4
6
ß
m
ß
8
ß
10
50
,_,_,_,_,_,_,_,_,_.,
ßßßß;•m
o t•mm• -•"
"ß .-•
I
12
i
14
1'6
GOLD (PPM)
0
Q,-m__ßß m•ß
2
4
ß
6
8
10 12
GOLD (PPM)
14
FIG. 6. A. Correlationplot of telluriumand gold assays
in areasof advancedargillicalteration.
Telluriumassays
arefrom10-mcoreintervals
andgoldassays
are 10-maverages
of 2-mcoreinterval
assays.
Line represents
the Te/Auwt ratioin calaverite.
Assays
underthe detectionlimit of 10 ppm
forTe isshown
as0 ppm.B.Silverversus
goldwithinareas
ofadvanced
argillicalteration.
Silverassays
are 10-m averagesof 2-m coreintervalassays.
18
20
MONTENEGROAu-AgDEPOSIT,DOMINICANREPUBLIC
1747
measuredvein densitydatashowno significantvariationwith rocktype (spiliteor sediment),depth,or
hole location,althoughthey are slightlyhigherin silicified rock (4.9%) than in nonsilicifiedrock (3.6%).
The averagecalculatedwall-rockgradeusingeq (1)
for the four holesis 2.3 ppm Au and 13.2 ppm Ag,
givingan Ag/Au ratio of 5.7. The higherAg/Au ratio
in the veinsreflectsthe presenceof Ag-bearingsulfosaltsin the veins and their general absencein the
wall rock.The estimatedAu wall-rockgradeof 2.3
ppm (for the 4 holes)is supportedby assaysof nonveined2-m intervals(fromthe 4 holes),which have
an averageAu gradeof 3.0 ppm. The estimatedAg
wall-rockgradeof 13.2 ppm (for the 4 holes)is not
aswell supportedby assays
of nonveined2-m intervals
(from the 4 holes),which have an averageAg grade
of 7.8 ppm.
Attemptswere madeto correlateestimatedwallrock gradewith alterationtype. Estimatedwall-rock
grades(0.6 ppm Au, 3.5 ppm Ag) for an intervalin
DDH195 that containsthe stageI alunitezonewith
minimalstageII overprintingare low and indicate
that little Au or Ag wasdepositedduringformation
of the stageI alunitezone(Fig. 4B, D, E). Estimated
wall-rock gradesin intervalswith well-developed
stageII pyrophylliteformationin the four holesare
alsolow (<2 ppm Au, <10 ppm Ag). Assaysof hand
samplesof massivepyrophyllite alteration, average
0.55 ppmAu (range< 0.1-3.5 ppm, 14 analyses)
and
2.5 ppm Ag (range < 0.1-11.8 ppm), supportthe
wall-rockgrade estimatesand confirmthat little, if
any, mineralizationoccurred during formation of
stageII massive
pyrophyllite.Althoughremobilization
of gold could have occurredduring stageII pyrophyllite formation,petrographicevidencesupporting
equilibriumbetweenpyrophylliteandpreviously
positedstageI pyrite (whichhostsstageI gold)argues
againstthis process.
Wall-rock Au and Ag, therefore, either was depositedduringformationof the stageI kaolinitczone
or duringformationof the stageII silicacaps.Based
on observationsof electrum as inclusionsin pyrite
that is coevalwith stageI kaolinitc,the poor correlationbetweensilicification
andpreciousmetalgrade,
and estimatedwall-rockgradesof 2.1 ppm Au and
15.2 ppm Ag in spilite (Table 2) where stageII silicificationis limited, it is mostlikely that mostdisseminated Au and Ag were depositedduring formation
of the stageI kaolinitczone.
GeochemicalEnvironment during Advanced
Argillic Alteration and Ore Formation
Calculationof elementmobility and water/rock
ratios
o
Chemical analysesand density measurementsof
spilites(Table 3) were usedto estimatethe amount
1748
MUNTEAN,KESLER,RUSSELL,AND POLANCO
TABLE
Sampleno.
SiO•
TiO2
A12Oa
3.
Abundancesof Major Element Oxides(wt%) in RepresentativeSamples
I
2
3
4
5
6
7
8
9
10
11
12
13
56.64
0.72
15.83
55.91
0.68
15.33
52.95
0.62
14.05
45.53
0.52
13.04
64.94
0.47
15.64
57.19
0.68
17.89
47.83
0.91
31.97
71.61
0.36
6.95
54.58
0.37
18.19
64.15
0.46
15.63
58.77
0.51
15.85
57.61
0.51
18.67
52.77
0.53
15.69
13.47
8.22
11.78
8.34
8.52
12.34
11.36
12.25
0.03
Fe•Oa]
7.74
9.14
8.42
5.96
9.23
MnO
0.13
0.23
0.52
0.01
0.01
0.01
0.01
......
MgO
4.24
4.72
5.86
0.01
0.01
0.03
0.08
0.05
0.05
0.08
0.04
0.07
CaO
3.45
3.66
5.11
0.08
0.09
0.13
0.14
0.08
0.10
0.10
0.08
0.09
0.08
Na•O
K•O
P•O5
4.02
1.12
0.09
4.91
0.95
0.09
0.05
0.83
0.09
1.49
1.39
0.08
0.01
0.04
0.15
-0.04
0.12
0.01
0.15
0.16
0.38
0.04
0.07
0.85
1.32
0.17
0.21
0.02
0.15
0.08
0.02
0.15
0.05
0.30
0.15
0.74
0.95
0.09
LOI
5.49
3.63
10.51
22.19
8.71
10.70
11.34
7.63
15.40
8.87
11.23
10.29
16.36
99.47
99.25
99.01
97.81
98.29
98.57
98.56
96.40
99.37
98.19
99.07
99.10
99.49
Total
Density•
2.80
2.77
2.84
2.99
3.14
3.10
2.82
2.85
3.13
3.04
2.95
3.01
2.99
Sampledescriptions:
Regionalsamples:1 = RD-73-383, spilite, 1.5 km N of Monte Negro orebody(UTM coordinates:
97,000N,
75,400E), representsregionalpropyliticalteration,unaffectedby PuebloViejo alteration;2 = RD-73-538, spilite, 3 km N-NW of
Monte Negroorebody(UTM coordinates:
99,400N, 73,400E), representsregionalpropyliticalteration,unaffectedby PuebloViejo
alteration;Monte Negro samples;3 = DDH206, 306 m, spilite, cc-chl-qtz-ill-pyalteration,4 m outsideadvancedargillic alteration;
4 -- DDH195, 247 m, spilite, stageI qtz-(nat)alun-pyalterationwith ca. 5% stageII overprint;5 = DDH195, 200 m, spilite, stageI
qtz-kao-pyalterationwith >50% stageII pyrophand <10% stageII silica;6 = DDH206, 302 m, spilite, stageII pyrophreplacing
qtz; 7 -- DDH206, 220 m, spilite, stageII massivepyroph; 8 = DDH195, 90-100 m, spilite, stageII silicareplacingstageI kao; 9
= DDH195, 116-126 m, spilite, stageII silicaand pyroph overprintingstageI kao zone; 10 -- DDH195, 154-162 m, spilite, stage
II pyroph and minor silicaoverprintingstageI kao zone; 11 -- DDH195, 204-214 m, spilite, stageII pyroph overprintingstageI
kao zone; 12 = DDH195, 218-224 m, spilite, contactbetween stageI kao and alun zoneswith ca. 50% stageII pyroph formation;
13 -- DDH195, 238-248 m, spilite, stageI alun zone with ca. < 25% stageII overprint
Abbreviations:alun -- alunite;cc -- calcite, chl = chlorite, ill = illitc, kao -- kaolinitc,LOI = losson ignition, py = pyrite, pyroph
= pyrophyllite, qtz = quartz, nat -- natro; -- -- not detected
Sampleswere analyzedby X-ray fluorescencespectrometry(XRF) usingglassdiscsand rhodiumX-ray excitation;water, sulfur,
and carbonateconstitutelosson ignition;the low totalsfor somesamplesmay representincompletelossof sulfur(both sulfideand
sulfate)on ignition
] Fe•Oa is total iron asFe•Oa
• Densitiesare graindensities(g/cma) measuredusingthe ASTMboilingmethod;assumed
to equalbulk density,becausethe
observedporosityof the samplesis negligible
of elementsmobilizedduringadvancedargillicalteration at Monte Negro. The compositesamplesused
for thesetestsrepresenta succession
of superimposed
alteration types, which provide informationon element mobility at a larger depositsize scale.Relative
gains and lossesof elements during alteration are
commonlybasedon the assumption
of constantvolume or constantmassof a specificelement (Gresens,
1967; Brimhall and Dietrich, 1987; Kerrich et al.,
0.6
0.4
0.2
0.0
1987). The bestevidencefor constantvolumeduring
alterationat Monte Negroisthe preservationof orig-0.2
inalphenocrystshapesandspatialrelationships
in the
ALTERED COMPOSITE SAMPLES
ALTERED HAND SAMPLES
altered spilite (Fig. 5F). Volume increasesof only 1
-0.4
to 5 percentcanbe attributedto sulfideveining(based
8
9
10
11
12
13
3
4
5
6
7
onvein densitystudiesof hole DDH195). In contrast,
calculations
assuming
constantTi, whichis the eleSAMPLE NUMBER
ment that appearsmostlikely to havebeen immobile
basedon petrographicobservations,
requirevolume
I
SiO2 •
AI203 t • i"-•l Fe203
increasesup to 190 percent,which cannotbe accomFIG. 7. Bargraphshowingadditionsandsubtractions
of SiO2,
modatedby the observedrock textures.
AlcOa,andFe•Oa(ingrams,relativeto 1 cmaof"startingspilite"),
Relativegainsand lossesof silica,aluminum,and in hand samplesand compositesamplesof altered spilite (Table
ironforthe alteredsamples,
calculatedassuming
con- 3), assumingconstantvolume during alteration. Alteration types
stantvolume,are shownin Figure 7. In generalsilica for the samplenumbersare given in Table 3.
MONTENEGROAu-AgDEPOSIT,DOMINICANREPUBLIC
was added to the rocksduring alteration,although
sample7 (massivepyrophyllite) showsa significant
lossof silica,whichis consistent
with the replacement
of quartz during formationof pyrophyllite,as indicated by petrographicobservations(Fig. 5C). Aluminumis oftenconsidered
to be immobileduringhydrothermalalteration(MeyerandHemley,1967),but
thisis obviouslynot a validassumption
in acid-sulfate
systems.Sample8, which is characteristicof stageII
silicification,is substantiallyenrichedin silicawith a
concomitantdecreasein aluminum,supportingthe
petrographicobservation
thatsilicareplacedkaolinitc
in the silicifiedzones(Fig. 5E). Aluminumaddition
is expressed
by the formationof pyrophyllite.Much
of the increasesin iron that apparentlytook place
duringalterationmightbe due to pyrite veining(especiallyin the compositesamples).Sample7 indicates
iron wasleachedduringthe formationof massivepyrophyllitealthoughno pyrite destructivetexturesare
seenin thin section.Magnesium,calcium,sodium,and
potassium,for the most part, were completelyremovedduring advancedargillic alteration.
Minimum
concentrations
in the fluid that would be
TABLE4.
1749
Isotopic Compositionof Pyrite and Natroalunite
Monte Negro
(%0)
Pyrite
Natroalunite
•vy_nal
Temperature•
Moore
(%0)
-9.5
-3.6
+20.0
+21.6
29.5
184øC
25.2
222øC
•The aqueoussulfide-sulfatefractionationfactor of Ohmoto
and Lasaga(1982) and the pyrite-aqueoussulfide fractionation
factorof OhmotoandRye (1979) were used;fractionation
between
aqueoussulfate and sulfate in alunite was assumedto be zero
(Kusakabeand Robinson,1977; Ohmoto, 1986)
respect to amorphoussilica at 170øC and requires
supersaturationwith respectto quartz at 300øC. Wa-
ter/rock ratioswere probablysomewhathigher,becauseit is unlikely that fluids could maintainsuch
high silicaconcentrations
duringthe entire life of the
hydrothermalsystem.These high water/rock ratios
(100-1,000) require minimum average aluminum
concentrationson the order of 10 to 1,000 ppm and
minimumaveragegold concentrations
on the order
of 0.1 to 10 ppb (Fig. 8).
required to explain the masstransferfor silica, aluminum,and gold canbe calculatedfor variouswater/
rock ratios(Fig. 8). Averagegainsand lossesof the
compositesampleswere used, becausethey reflect Temperaturesand chemistryof the hydrothermal
elementalmasstransferon a depositscale.A water/ fluids
Fluid inclusionand isotopicconstraints:Possible
rock ratio of 100 requiresa minimum averagesilica
concentrationof 740 ppm and silicasaturationwith primary fluid inclusionsfrom silicified spilite and
quartz in sulfide veins from Monte Negro homogenizedat temperaturesof 102ø to 136øCandyielded
salinitiesof 0.8 and 3.8 equivwt percentNaC1(Potter
et al., 1978). These resultsare comparableto data
from the Moore orebody,which yielded homogenizationtemperaturesrangingfrom 135ø to 195øC and
approximatelyzero salinities(Kesler et al., 1981).
Becausemuchof the quartzin the silicifiedcapsand
veins at Monte Negro is probably recrystallized,
I
I• QUARTZ
SAT.
300oc
•
however, these data probablydo not representthe
conditionsof the fluid duringthe originaldeposition
of silica (Fournier, 1985; Sanderand Black, 1988),
o
and
it is necessaryto use additionalobservationsto
I
I
I
estimatemineralizingtemperatures.
,
i
I
Sulfurisotopethermometryon the alunite-pyrite
3
2
I
0
-1
intergrowth
shownin Figure 5A indicatesa temperLOG WATER:ROCK MASS RATIO
ature of 184øC for the stageI alunitezone. A temFIG. 8. Estimatedwater/rockratiosfor Monte Negroore fluids. peratureof 222øCwascalculated
fromthe &34S
values
The three solidlinesshowhow estimatedminimumaveragecon- reportedby Kesleret al. (1981) for a similaralunitecentrationsrequired to explain masstransferof SiO2, A1, and Au
pyrite equilibriumintergrowthfrom the Moore orevary with water/rock massratio. The concentrationswere calbody
(Table 4). Althoughkineticsof sulfur isotope
culatedby determiningthe averagegainor lossof a givenelement
relativeto a cma of rock,assuming
a rockdensityof 3 g/cc.Con- exchangebetween sulfideand sulfateinhibit the atcentrationsandwater/rockratiosrequiredto explainmasstransfer tainment of equilibriumbelow 200øC in mostgeoof Fe, Mg, Ca, Na, and K are similarto thoseof aluminum.The chemicalenvironments,
extremelyacidandhigh suldashedhorizontallines refer to silicasaturation(sat.) concentrafur
conditions
such
as
prevailed
in the alunite zone
tionsfor quartzandamorphous(amor.)silicafor 150ø and300øC,
whereasthe dashedverticallinesrefer to the water/rockratios permit equilibriumto be attainedat temperaturesas
required for silica addition at these silica concentrations.
low as150øC (OhmotoandLasaga,1982). If sulfide-
2------•
AMOR.
SILICA
SAT.
150øC
1750
MUNTEAN,
KESLER,
RUSSELL,
ANDPOLANCO
sulfateequilibriumwasnotattainedat MonteNegro, temperatureattainedfrom pyrite-alunitethermomthat moststageI althe 184øC representsa maximum,sinceattainment etry, this observationsuggests
of equilibriumwouldresultin a largerfractionation teration formed at about 200øC.
The coexistenceof pyrophylliteand diasporein
and a lower temperature.
Constraintson temperatureand coolingpathsfrom stage II alteration limits temperaturesto between
phaseequilibria:Additionalinsights
intomineralizing 285ø and 320øC (Fig. 9, pattern 3). Althoughdiatemperatures
canbe obtainedfromphaseequilibria sporecanformat lower temperaturesin equilibrium
in the A12Oa-SiO2-H20
system(Hemleyet al., 1980; with kaolinitc, no suchtexture was observed.The reFig. 9). Equilibriumbetweenkaolinitcandquartzin placementof quartzby pyrophylliteunder quartzconditionsconstrains
temperaturesto
stageI alterationis limited to temperaturesbelow undersaturated
a
similar
range
(260ø-340øC,
pattern
4). Thus,it ap260øC, where quartz-saturated
solutionsreact with
kaolinitcto formpyrophyllite(Hemleyet al., 1980). pearslikely that pyrophylliteformationduringstage
Kaolinitccan existup to temperaturesof 285øC if II alterationand mineralizationformedat temperathe fluidis undersaturated
with respectto quartz,al- turesof about300øC,slightlyhigherthantemperathoughthe previouslydescribedsilicatexturesfor turesestimatedfor stageI. An uppertemperatureof
stageI suggestsolutionswere supersaturated
rather 220øC for the formationof metastablesilicaphases
thanundersaturated.
Therefore,stageI quartz+ ka- suchas a cristobaliteand amorphoussilicacan be
of active geothermal
olinitcprobablyformedundertemperature
conditions estimatedfrom observations
shownin Figure9 by the bandabovethe quartzsat- systemswhere quartz, not metastablesilicaphases,
uration line (pattern 2). Combinedwith the 184øC hasbeen foundto controldissolvedsilicain all geothermal reservoirwaters above 180øC (Arnorsson,
1975; Fournier,1985) andabove220øC undervery
acid conditions(J. W. Hedenquist,pers. commun.,
-1.5
1988).
A PYROPHYLLITE
....,.....
AscendingstageII fluidsprobablybecamequartz
undersaturated
asthey cooledthroughthe regionof
retrogradequartzsolubility,shownin Figure9 where
E
the quartz saturationline bendsto a negativeslope.
A rapidlycoolingsolutionmightnothavemaintained
O
-2.5
equilibriumsaturationwith respectto quartzif dissolution rates were slow relative to the rate of fluid
-3.0
150
200
250
300
350
-1.5
: .-: :. ' . :: ' MIXtN•
. •.•-.•'" .:.
•O -2.5
150
1
•
200
250
300
3•
TEMPERATURE
(øC)
S•geI,Pyrite-•unite
Sulfur
Isotope
ThermomeEy,
T=184C
2 •
•age I, Kaolinitc-QuaY,
T<260C
3
4
S•ge II, Dias•e coexl•ingwithPy•hyllite, 285C<T<320C
Sage II, Pyr•hyllite, silicaundersaturated
fluids,260C<T<340C
•
•
migration(Hemley et al., 1980). Sucha fluid would
moveto theleft of thequartzsaturation
line,through
the andalusitefield, into the diasporefield andeventuallyintothe pyrophyllitefield (Fig. 9B). Andalusite
has not been observedat either Monte Negro or
Moore,althoughit shouldbe presentat depth.It is
a commonalterationmineral in the gold depositsof
the Carolinaslatebelt (KleinandCriss,1988) andat
the Equity silverdeposit(WojdakandSinclair,1984),
whichare characterized
by pyrophyllitealteration.
As upwellingstageII fluids cooledin the pyrophyllitefield, acidicconditionswouldhaveinhibited
silicaprecipitation(Fournier,1985) causingsilicasupersaturation(with respectto quartz,chalcedony,a
cristobalite,
andpossibly
evenamorphous
silica).This
couldhavebeenmaintained
untilthe systemwasperturbed. Munteanet al., (1988) have suggested
from
•i]80datathatmixingbetweenmineralizing
solutions
andgroundwaterin the coarse-grained
sediments
that
overliethe spilitecouldhavebeensuchanevent.Note
in Figure 4C that the silicifiedcapsare concentrated
FIG. 9. A. Ph•e relationsin the A1•O3-$iO•-•O system in the sediments
directlyoverlyingthe spilite.
(Hemleyet al. 1980). Amo•hous silicaand a cristobalite•tuConstraintsonfluid chemistryfrom phaseequilib5 '.•
6 •
S•ge II,Pyr•llite, silica
supersaturated
flui•
S•geII,Siliciflca•,m•s•ble silica,
T<220C
rationlines•e taken fromHelgesonet al. (1978). B. Patte•s
showing
temperature
regions
for•terationtypesM MonteNegro.
Boldline with •row showsprobablecoolingpath for stageII
fluids.
ria: Phaseequilibria,depictedon a logfo•-pH diagram,providesfurtherconstraints
on fluidchemistry
(Fig. 10). For instance,for stageI fluidsat 200øC,
MONTENEGROAu-AgDEPOSIT,DOMINICANREPUBLIC
1751
;
A
-34
B
-26
STAGE II
',
T=300
" •'
•'S=0.01m
"
d•
K=0.001
:
/
oc
X,x,c• K=0.01
I
-36
o
-38
,
z
o
•:
0
-4O
0
1
3
4
5
0
1
pH
2
3
4
5
pH
FIG. 10. A. pH-fo• diagramconstructedto representstageI at 200øC, vaporsaturationpressure,
quartz saturation,•;mS = 0.01, mK+ -- 0.1, and an ionic strengthof one. The compositionof stageI
fluidsin the alunite zone is the dark band, which representsa sulfate/sulfideratio of 0.5. Substitution
of natroalunite(mNa+ = 1.0) for alunitein thisdiagramwould shiftphaseboundariesby lessthan0.2
pH andlogfo• units.The cross-hatched
arearepresents
the kaolinitczone.The verticaltotal aluminum
contoursrefer to the totalconcentration
of aluminumin the fluid in equilibriumwith kaolinitc.Aluminum
species,
includedin the calculation,
areAI+3,AI(OH)+2,andAI(OH)• (concentrations
of AI(OH)• and
Al(SO4)+ are insignificant).The phaseboundarieslabeled "K = 0.001" representthe kaolinitcfield
betweenthe muscoviteand alunitefieldsin the sulfatedominantregionat a K+ concentrationof 0.001
m. B. A PH-fo2diagramconstructedto representstageII at 300øC, vaporsaturationpressure,quartz
saturation,•mS -- 0.01, mK+ -- 0.1, andanionicstrengthof one.The shadedareaisthe inferredregion
for stageII pyrophyllite-formingfluids.The vertical total aluminumcontoursrefer to the total concentrationof aluminumin the fluid in equilibriumwith pyrophyllite.The phaseboundarieslabeled"K
-- 0.01" representthe pyrophyllitefield betweenthe muscoviteandalunitefieldsin the sulfatedominant
region at a K+ concentrationof 0.01 m. Data usedin constructingthe diagramis shownin Appendix A.
total sulfurconcentrationsgreaterthan 0.01 m cause bach, 1987; and Stoffregen,1987). It is unlikelythat
too large a stabilityfield for native sulfur. At total other sourcesof sulfur, suchasseawatersulfate,could
sulfur concentrations less than 0.01 m, the alunite have producedthe extremely acid conditionsobfield shrinksinto the aqueoussulfatedominantfield, servedin these deposits(Seyfriedand Mottl, 1982;
whichdisagrees
with estimates
of sulfate/sulfide
ratios Bowerset al., 1985) without the introductionof exin the fluids, as discussedbelow. A total sulfur con- ternally derivedacid constituents
suchas magmatic
centrationof 0.01 m wasalsoestimatedfor stageII. SO• and H•S. Isotopicanalysesof pyrite at Monte
At total sulfur concentrations
less than 0.01 m un-
reasonably
highiron concentrations
(20,000 ppm)are
requiredto maintainpyrite-pyrophylliteequilibrium.
At total sulfur concentrationsgreater than 0.03 m,
the nativesulfurfieldexpandsgreatlyandwouldhave
becomea dominantphase.
Sulfate/sulfideratiosof the stageI ore fluid can
alsobe estimatedfrom S isotopedata,if the bulk isotopiccomposition
of the sulfuris known.It is widely
acceptedthat the sourceof the acidityand mostof
the sulfur in hypogenealunite zonesis magmatic
(BrimhallandGhiorso,1983;Bethke,1984; Giggen-
Negrofall largelywithina •i34Srangeof 0 and-10
per mil (Kettler, 1989), supportingthe suspected
magmaticsulfursourcefor this system.If the sulfur
sourceisassumed
to be largelymagmaticwith anisotopic ratio of 0 per mil, a sulfate/sulfideratio of 0.5
canbe calculatedfromthe sulfurisotopecompositions
of the pyrite-alunitepair listed in Table 4 (Ohmoto
and Rye, 1979, p. 530). The sourceof acid constituents and sulfur during stageII was also probably
magmaticgases,althoughthe lack of aluniteandrelatively reduced conditionssuggesta decreasein the
amount of SO•.
1752
MUNTEAN,KESLER,RUSSELL,AND POLANCO
The upperpH for the stageI alunitezoneislimited Geochemicalmodelingof gold transportand
by the alunite-muscovite
phaseboundary(pH -- 4), deposition
Modelingparameters:The observationsdiscussed
andthe lower pH is limited by the alunite-nativesulfur phaseboundary(pH = 1.5). The kaolinitc-mus- above indicatethat advancedargillic alterationand
covite phaseboundary(pH -- 2.9) is the upper pH preciousmetalmineralizationwere contemporaneous
limit for the kaolinitc zone. The lower pH is con- at Monte Negro. It followsthat preciousmetalsare
strainedby contoursof total aqueousaluminumcon- depositedby the samefluidsthatformedthe advanced
centrationthatwouldbe in equilibriumwithkaolinitc. argillicalteration.This conclusion
contrasts
with sugThe 10-ppmisopleth(pH = 2) waschosenasthe up- gestions
that preciousmetalsare introducedinto acidper aluminum concentrationlimit, since aluminum sulfatesystemswhen near-neutral,chloride ground
appearsto have been largely immobileduring stage watersfloodthe systemafter collapseof the acid-sulI alteration,asindicatedby petrographicobservations fate system(Stoffregen,1987; Berger and Henley,
showingresidualaluminumremainingin phenocryst 1989).
In order to further evaluatethe capacityof acidsitesaskaolinitc.The upperfo•limit forthekaolinitc
zone is the kaolinite-alunitephaseboundary,while sulfatefluidsto transportand depositgold, we have
the lower fo• limit would be the pyrite-pyrrhotite usedthe computerprogramsCHILLER andSOLVEQ
phaseboundary(not shownin Fig. 10A).
(Reed, 1982; Reed and Spycher,1985; Spychefand
A pH rangeof 0.9 to 1.6 for pyrophylliteformation Reed, 1989) to modelore-formingprocesses
at Monte
at 300øC during stage II is constrainedby total Negro. Specificgold-depositing
processes
that were
aqueousaluminumconcentrations
of 1,000 to 10 ppm modeled include sulfidationof wall rock during dethat wouldbe in equilibriumwith pyrophyllite,which positionof stageI disseminated
gold mineralization,
isthe rangeof minimumaluminumconcentrations
re- and boiling, fluid mixing,and coolingto accountfor
quiredto explainthe aluminumadditionthatoccurred gold depositionin stageII veins.Wall-rock reaction
duringpyrophylliteformation.The upperfo• limit wasnot modeledfor stageII becauseof the previously
for stageII fluids is the alunite-pyrophyllitephase mentioned lack of alteration selvagesaround the
boundary.Althoughthe lowerfo• limit is the pyrite- veins.It shouldbe kept in mindthat thesecalculations
pyrrhotite phaseboundary,the lack of texturesin- do not constitutea comprehensiveevaluationof all
dicatingreplacementof aluniteby pyrophylliteis ev- possiblemineralizingprocessesin acid sulfate deidencethat the pyrophyllite-forming
fluidswere close positsand are, instead,an effort to quantifyand furto equilibrium with alunite and therefore near the ther test processes
that were indicatedby our observations at Monte Negro.
alunite-pyrophyllitephaseboundary.
TABLE5.
Estimated Initial Fluid Compositions
Component
•
species
Stage I
Stage II
Rationale
Temperature
pH
200øC
2.5
300øC
1.5
Sulfur isotopethermometry, phaseequilibria
Phase equilibria
Logfo2
Logfs2
-38.3
-6.6
-28.9
-4.2
Calculated
Calculated
I
0.93
CI
SO4
HCO3
HS
0.76
Calculated
33,123 ppm (1.0 m)
8.9 ppm (0.0001 m)
10,000 ppm
309 ppm (0.01 m)
33,172 ppm (1.0 m)
99 ppm (0.001 m)
10,000 ppm
309 ppm (0.01 m)
Assumption
Phaseequilibria
Fluid inclusiongasanalyses
Phaseequilibria
SiO2•
172 ppm
401 ppm
Quartzsaturation
AI
Ca
Mg
0.05 ppm
250 ppm
250 ppm
20 ppm
250 ppm
250 ppm
Phaseequilibria
Masstransfer
Masstransfer
Fe
1.6 ppm
5,227 ppm
Pyrite saturation
K
Na
3,653 ppm (0.1 m)
19,687 ppm
3,659 ppm (0.1 m)
11,330 ppm
Assumption
Charge balance
Au2
6.9 ppb
119 ppb
Goldsaturation
1 Concentrations
of componentspeciesrefer to total concentrations
• Telluriumconcentrations
of ca. 4.2 X 10-•l ppm at 200øCand6.4 X 10-8 ppm at 300øCsaturatecalaverite,giventhe listed
goldconcentrations
andfo• valuesare assuming
H•Te asthe dominantaqueoustelluriumspecies(Ahmadet al., 1987).
MONTENEGROAu-AgDEPOSIT,
DOMINICANREPUBLIC
Total sulfurconcentration,
sulfate/sulfide
ratio,and
pH usedfor the model calculations(Table 5), were
STAGE
17 5 3
I
__ppm
derivedfrom thefo2-pH constraints
(Fig. 10), with
the fluidsbeing constrainedto the kaolinitcand pyrophyllitestabilityfieldsfor stagesI and II, respectively. Total aluminumconcentrations
were determined by saturationwith kaolinitc (stageI) and pyrophyllite(stageII). Ca andMg wereconstrained
from
minimumconcentrations
requiredto explainthe mass
transferof theseelementsduring alteration(Fig. 8).
-3 T -- 200oc
pH -- 2.5
Total C1 and K concentrations were estimated at 1
and 0.1 m, respectively,as in the fo2-pH diagrams.
-2
The concentrationof total carbonate(10,000 ppm)
wasbasedon analysesof COs in 1-g samplesof vein
quartz (Kesleret al., 1986). Silicawasdeterminedin
both fluidsby equilibrationwith quartzbecausethere
is petrographicevidenceof quartz-kaoliniteequilibrium during stageI, and stageII fluidspassedfrom
conditionsof silica undersaturationthrough quartz
saturationto silicasupersaturationat approximately
300øC. The Na content was determinedby charge
-1
0
1
LOG mCI
STAGE
o
II
.¾•
balance constraints in the solutions. Na was used for
this purposerather than C1becausewall-rockalterationservedasboth a source(alterationof albite) and
-3 T -- 300o C
sink (formationof natroalunite)for Na, whereasno
pH --- 1.5
LOG fO2 -- -29
geologicallyreasonablesourceor sinkfor C1 could
-4
be identified.Total Fe and Au were fixedby equili-2
-1
0
1
brationwith pyrite and gold, respectively.The relaLOG rnCI
tively high Fe contentindicatedfor stageII is supFIG. 11. Relative importanceof H2S and C1to gold solubility
ported by evidencefor pyrite stabilityduring stage at Monte
Negro. SolidcontourslabeledAuc]/AuHs
showratiosof
II pyrophylliteformationand Fe assays
ashigh as30 log molality AuCl•/log molality Au(HS)•, and dashedcontours
percent in 2-m drill intervals.Copper and zinc were
excludedfrom the calculationsfor simplification,althoughthe fact that they form solublechloridecomplexes (Barnes, 1979) suggeststhat their behavior
would be similar to that of iron.
The relative importanceof the main gold-transportingcomplexes,
Au(HS)• andAuCI•, withchanges
in sulfide and chloride concentrations, is shown in
showvariation in total dissolvedgold. Shadedregionsrepresent
the conditionsinferred for stageI and II fluidsand representkaolinitc- and pyrophyllite-formingfluids, respectively(Fig. 10).
Limits on H2S are discussedin the text; upper mc]- limit is taken
from maximumsalinities(5.3 m; Heald et al., 1987) observedin
fluid inclusionsfrom other acid-sulfatedeposits.Gold solubility
dataare fromHelgeson(1969) andShenbergerandBarnes(1989).
Ionic strengthwas assumedto equal mc•-. Under the conditions
listed, H•S is the dominantaqueoussulfurspeciesfor both stages
I and II.
Figure 11 for conditionsrepresentingstagesI andII.
The plotsshowthat duringstageI AuCI• couldhave
becomethe principal aqueousgold speciesonly at
improbablylow sulfideconcentrations
(<0.001 m) testthispossibility,we reactedthe stageI fluid (Table
and high salinities(>3 m). Under the low pH (1.5), 5) with spilite having the average compositionof
high temperature(300øC) conditionsof stageII, analyses1 and 2 in Table 3. In the calculation,spilite
however, AuCI• probably predominated over wasaddedto the stageI fluid at 200øC in increments
Au(HS)•, even to low salinities(mcl- = 0.01). Al- of 0.1 g until advancedargillic alteration ceasedto
whichwere carriedout for anopen
thoughother aqueousgold species,suchas AuHSø, form.Calculations,
mighthavebeenimportantat low pH, thermodynamic system,
showthat3.0 X 10-6 g of goldweredeposited
data to evaluatetheir role at high temperaturesare to producea rock grading2.9 ppm Au andcontaining
lacking.
an average alteration mineralogyof 41.1 percent
Reactionprogressmodelsand resultsfor stageI: quartz, 19.7 percent pyrite, 35.0 percent kaolinitc,
As discussed
above,petrographicand assaydata sug- and 4.2 percent muscovite.Advancedargillic altergest that sulfidationof iron-bearingwall-rockmin- ation ceasedto form (muscoviteformed insteadof
erals,includingsiderite,duringthe formationof the kaolinitc)after 1.1 gramsof spilitewere addedto the
kaolinitc zone during stageI causeddepositionof fluid at a fluid/rockmassratio of 973, similarto the
large amountsof low-gradegold (Kettler, 1989). To high ratiosdeducedabovefrom wholerock analyses.
1754
MUNTEAN,KESLER,RUSSELL,
AND POLANCO
Pyrite formedby the sulfidationof ferrousiron by
the reaction:
4FeO + HSO• + 7H2S+ H + = 4FeS2+ 8H20. (2)
ceasedto precipitate, however, becauseincreasing
pH stabilizedthe goldbisulfidecomplexto a greater
degreethanthe decreasing
total sulfurconcentrations
destabilized
it.
Geologicmodelsand resultsfor stageH: CalculaThe consumption
of hydrogenion,sulfate,andsulfide
in thisreactioncausedanincreasein pH anddecreases tionsto comparethe efficiencyof cooling,fluid mixin the oxidationstateand concentrationof aqueous ing, and boiling as causesof gold depositionin stage
sulfide(Fig. 12). The oxidationstate decreasedbe- II veins was also carried out for open systems,alcausesulfurwasconsumedat a sulfate/sulfidemolar thoughthe boiling calculationwas closedto escape
ratio of 1/7 in eq (2), whereasthe initial stageI fluid of gases.Fluid mixing was modeledby mixing the
had a ratio of 1/100 (Table 5). The decreasein 300øC parent fluid with groundwater represented
aqueoussulfideandoxidationstatecausedby the re- by pure water at 25øC. Quartzhasnotbeenobserved
(e.g.,notallowed
actionin eq (2) causedgoldto precipitateby the sul- in stageII veinsandwassuppressed
to precipitate)in all the model runs.This is further
fidation-reduction reaction:
supportedby experimentaldata indicatingquartz
16FeO + 8Au(HS)• + 3HSO• + 13H•S + 11H+
precipitatesvery slowlyin acid solutions(Fournier,
1985), as discussedabove. Other metastablesilica
-- 16FeS• + 8Au + 28H•O. (3) phaseswere suppressed
above220øC, the previously
noted
upper
temperature
limit of thesephasesin acThisreactionconsumedsulfurat a sulfate/sulfidemotive geothermalsystems.
lar ratio of 3/13, resultingin gold depositionaccomResultsof modelingthe stageII fluid indicatethat
paniedby a sharpdecreasein the oxidationstate(Fig.
although
all three processesdepositedgold in the
12). With continuedreaction with wall rock, gold
veins,they producedistinctivevein mineralogies
and
textures.For instance,coolingand mixingwould depositlarge amountsof pyrite in the veins(85.1% for
mixing and 67.8% for cooling) and essentiallythe
LOG fO2
-38
sameamountof gold(9.3 X 10-5 g), indicatingthat
coolingplayeda moreimportantrole thandilutionof
chloridein causinggoldprecipitation.Mixingdid not
causesignificantdepositionof silica(thelackof which
is characteristicof the vein mineralogy)becausethe
fluid did not reachsufficientlevelsof supersaturation
with silica.In contrast,boilingprecipitatedonly gold
until 220øC, where chalcedonywasallowedto precipitate.Pyrite did not form duringboilingbecause
aqueoussulfidewas lost to the gasphase(Fig. 13).
AlthoughAuCI• wasthe dominantaqueousgoldspecies and the chloride contentincreasedwith boiling
(Fig. 13), golddepositionresultedfromthe decreased
solubilityof gold asthe temperaturedropped.Note
that pH decreasedduring boiling (Fig. 13). This cu-
-42
MOLALITY TOTAL AQUEOUS
0.011
SULFIDE
0.01
0.009
0.008
0.007
3.5
3
rious feature, which contrastswith the common view
thatpH increases
duringboilingby lossof CO2, which
is derivedby the followingreaction:
2.5
20 PPMGOLDINWALL
HCO• -t-H + -- H•O -t-CO2(g).
(4)
18
12
8
4
0
1.1
0.9
0.7
0.5
0.3
0.1
GRAMS OF ROCK ADDED
FIG. 12. Resultsof CHILLER run modelingsu]fidationduring
At a pH of 2.5, essentiallyall of the dissolvedcarbonateis in the form of H•CO3, and there is only a
negligibleamountof HCO• in the fluid,whichmakes
pH increases
dueto eq (4) insignificant
relativeto pH
decreasesdue to disassociation
of weak acidsduring
cooling(DrummondandOhmoto,1985; Reed, 1989).
These resultssuggestthat a combinationof the
stageI, showingchangein 1ogfo•,molalityof totalaqueous
sulfide,
modeledherewasrequiredto producethe
pH, and gold grade in wall rock as gramsof spilite are addedto processes
the stageI fluid (Table 5). The sulfidationrun wasstoppedafter observedgold-bearingstageII veins. Of particular
1.1 gramsof spilitewere addedwhen kaolinitcceasedto form.
importancein thisregardisthe observation
that much
MONTENEGROAu-AgDEPOSIT,
DOMINICAN
REPUBLIC
COOLING
...........
MIXING
.......
TOTAL MOLALITY AQUEOUS
1.6
BOILING
CHLORIDE
1755
widelythoughtto be the causeof hydrothermalbrecciasin otherepithermaldeposits(BergerandEimon,
1983; Hedenquistand Henley, 1985).
1.2
Important Factorsin Grade Development
at Pueblo Viejo
0.8
0.4
0
0.01
It is apparentfrom theseobservations
that several
factorswere importantin makingthe Monte Negro
orebodyeconomically,
attractive.The first,andpossiblymostimportantone,waslocalizationof the hydrothermalsystemin a maar-diatremecomplexthat
formedby explosivevolcanism.The maar-diatreme
complexundoubtedlyhad a high porosityand permeability, especiallyaroundits margins,that facilitated circulationof a shallowhydrothermalsystem.
Deeper diatremepermeabilitycouldhave provided
anavenueby whichgasesandothercomponents
from
subjacentmagmamixedwith thesewaters.Although
the quantitativeimportanceof suchmagmaticcontributionsisdifficultto estimate,Symonds
et al. (1987)
reportedthat gasesfrom the Merapi volcanoin Indonesia
maybe enrichedin goldby a factorof 104to
105relativeto the magma.
The second,third, andfourthimportantprocesses
are intimatelyrelatedandrepresentcriticalstagesin
the historyof mineralizationandalterationat Monte
Negro. The secondprocesswasclearlysulfidationof
wall rockandassociated
disseminated
golddeposition
during stage I advancedargillic alteration. Gold
gradesproducedby this processwere marginalfrom
an economicstandpoint,but they provide a much
betterbackground
thanbarrenrockto be cut by stage
II gold-bearingveins.For instance,usingthe datain
Table 3, Monte Negro, with all the veinsremoved,
TOTAL
MOLALITY
AQUEOUS
SULFIDE
....
••
-
0.008
...
0.006
0.004
0.002
0
1.8
pH
1.6
1.4
1.2
1
0.8
140o
1200
1000
•
800
600
400
/ GOLD
ONLY
':
20o
0
100
120
140
160
180
200
220
240
260
280
300
TEMPERATURE
FIG. 13. Resultsof StageII CHILLER runs,showingchange
in molalityof total aqueouschloride,molalityof total aqueous
sulfide,pH, andgoldgradein the veinswith a temperaturedecrease.Boilingwasmodeledasanisoenthalpic
process.
Only gold
wasdepositedin the boilingrununtil 220øC,at whichpointchalcedonywasallowedto precipitate.The humpin the goldgrades
in the mixingrun is due to a maximumof the pyrite solubilityat
ca. 170øC.The coolingrun produced9.3 X 10-s g of goldin a
vein consisting67.8% pyrite, 31.4% chalcedony,0.8% graphite,
and 101 ppm gold.The fluid mixingrun produced9.3 X 10-• g would still contain about 34.4 million metric tons of
of goldin a vein consisting
of 85.1% pyrite, 14.1% chalcedony,
0.8% graphite, and 135 ppm gold. The boiling run produced9.4
ore grading2.3 ppm (78 million g of Au). This im-
X 10-s gofgoldin aveinconsisting
ofessentially
100%chalcedony portantbackgroundconcentrationwasupgradedby
and 393 ppm gold.
the third importantprocess,which wasthe formation
of stageII veins.The data in Table 3 indicatethat
althoughtheseveinsmaycontainonly about2.7 mil-
gold in the Moore orebodyis foundalonggrowth lion metrictonsof ore, they havean averagegrade
zonesin pyrite, asdiscussed
above.The gold-bearing of 21.1 ppmAu andcontain57 milliong of Au, clearly
zonesapparentlyformedwhenpyrite wasnot being underscoring
their importancein makingMonteNedeposited,andthey accountfor muchof the goldin gro a world-classgolddeposit.The fact that stageII
the deposit.Accordingto the calculations
discussed took placeat a highertemperaturethan stageI sugare moreimportant
here,thisgoldcouldhavebeendeposited
by boiling, gestsfurtherthat waxingsystems
whereasmuchof the interveningpyrite mighthave thanwaningsystems
in developinggradeandtonnage
beendeposited,
with or withoutgold,by coolingand/ in acid-sulfatesystems.
Fluidsfromhighertemperaor mixing. Boiling during stageII could have been ture, waxingstagescancarry moregoldandhavethe
potentialto concentrateearlier, low-grademineralization. At Monte Negro, these stageII fluids also
remobilizedsilica,causingformationof the silicacaps,
which actuallymeritsrecognitionasthe final important processin developmentof mineablegrade at
Monte Negro. The silicacapsapparentlysealedthe
upperpart of the hydrothermalsystem,whichcaused
causedby hydrofracturingof the silicacap, which
wouldhavecauseda suddenreleaseof pressureand
flashingof the fluid under superisoenthalpic
conditions (Reedand Spycher,1985; Spycherand Reed,
1989). Self-sealing
andhydrofracturing
are common
in active geothermalsystems(Muffler et al., 1971;
White et al., 1971; Henley andEllis, 1983) and are
1756
MUNTEAN,KESLER,RUSSELL,AND POLANCO
hydrofracturing
andformationof the high-gradestage Berger,B. R., andEimon,P., 1983, Conceptualmodelsof epithermal preciousmetal deposits,in Shanks,W. C. III, ed.,
II veins. The capsalso protected the depositfrom
Cameron volume on unconventional mineral deposits: New
rapid erosionduringlater uplift and exposure.
York, Am. Inst. Mining Metall. PetroleumEngineers,p. 191205.
It followsfrom these generalizationsthat exploration for similarsystemsshouldfocuson areasof ex- Berger,B. R., andHenley, R. W., 1989, Advancesin the understandingof epithermalgold-silverdeposits,with specialrefplosive volcanismin relatively mafic volcanicrock.
erence to the western United States:ECON. GEOL. MON. 6, p.
The lack of magmaticmaar-diatremecomplexesin
405-423.
most basaltic terranes and the absence of iron-rich
rock in mostrhyoliticterranessuggests
that the most
favorable terranes would be andesitic island-arc vol-
canic zones.Within individual zonesof alteration, at-
tention shouldfocuson areaswith pervasive,rather
than vein-controlled, alteration in which evidence can
Bethke, P.M., 1984, Controls on base and preciousmetal mineralizationin deeperepithermalenvironments:
U.S. Geol.Survey Open File Rept. 84-890, 14 p.
Bowers,T. S., Von Damm, K. L., Edmond,J. M., 1985, Chemical
evolution of mid-oceanridge hot springs:Geochim. et Cosmochim. Acta, v. 49, p. 2239-2252.
Bowin,C. O., 1966, Geologyof the CentralDominicanRepublic:
Geol. Soc. America Mem., v. 98, p. 11-84.
Brimhall,G. H, Jr., and Dietrich, W. E., 1987, Constitutivemass
balancerelationsbetween chemicalcomposition,volume, den-
be foundfor wall-rockgoldvalues.Particularattention
shouldbe given to zonesin which successive
hydrothermal eventshave overprintedthis wall-rockgold
sity,porosity,andstrainin metasomatic
hydrochemical
systems:
with higher grade vein-hostedgold. If the form of
Resultson weatheringand pedogenesis:
Geochim.et Cosmochim. Acta, v. 51, p. 567-587.
alterationat PuebloViejo can be used as a general
guide, it shouldbe foundthat acid sulfatealteration Brimhall,G. H, Jr., and Ghiorso,M. S., 1983, Origin and oreformingconsequences
of the advanced
argillicalterationprocess
zonesexpandin size upward and that their size and
in hypogeneenvironments
by magmatic
gascontamination
of
grade are enhancedat areasof increasedporosity,
meteoric fluids: ECON. GEOL., v. 78, p. 73-90.
such as the spilite conglomeratecontact at Monte Drummond, S. E., and Ohmoto, H., 1985, Chemical evolution
andmineraldepositionin boilinghydrothermalsystems:
ECON.
Negro.
Acknowledgments
Thanksgo to RosarioDominicana,S.A., especially
Michael Seaward,for travel, field, and logisticalsupport at PuebloViejo. Specialthanksgo to Mark Reed
for hisgenerosityin allowingthe useof hiscomputer
programsSOLVEQ and CHILLER and for his commentson an earlier draft of the manuscript.Discussionswith R. Kettler, C. Alpers, R. Loucks,R. Seal,
A. Arribas, J. O'Neil, E. Essene,and J. Hedenquist
provedinvaluablein completingthisproject.Reviews
from two EconomicGeologyrefereesproved to be
very helpful.This researchwassupportedfinancially
by NationalScienceFoundationgrantEAR-8607021
awardedto SEK andPhillip Meyers.The seniorauthor
was supportedby a National ScienceFoundation
graduatefellowship.The scanningelectron microscopeand electron microprobeat the Universityof
Michiganwere made availableby NSF grantsBSR83-14092 and EAR-82-12764, respectively.
August 17, 1989; August 27, 1990
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APPENDIX
Thermodynamic
Data Usedto ConstructLogfo•-pHDiagrams
Log K
Reaction
200øC
300øC
Source
Aqueoussulfurspecies
1. H2S(aq)
q-202 q-Na+ = 2H+ q-NaSO•
2. H2S(aq)
q-202 = HSO• q-H+
68.01
71.3
49.48
54.31
1, 2, 3
1, 2
3. HgS(aq)
= H+ + HS-
-6.96
-8.06
2
4. HS- + 20•. + Na+ = NaSO• + H +
5. HSO• + Na+ = H + + NaSO•
74.97
57.54
-3.31
-4.74
1,3
1,3
Sulfur saturation
41.00
6. 2H•.S(aq)
+ O•.= 2S(t)= 2H•.O
7. 2H+ + 2HSO• = 302 + 2S(t)+ 2H•.O
Pyrite-hematite-magnetiteequilibria
8. 4FeS2 q- 150•. q- 8Na+ q- 8H•.O = 2Fe•.O3
+ 16H + + 8NaSO•
9. 4FeS•.q- 150•. q- 8H•.O = 2Fe•.O3 q- 8H +
+ 8HSO•
10. 3FeS•.q- 6H•.O = FeaO4 q- 6HS- q- 6H + q- O2
11. 4FeaO4 q- O2 = 6Fe•.Oa
31.26
-77.36
1,2,4
1,2,4
467.72
340.84
1,3
494.20
379.48
-97.35
31.01
1,2
-109.17
40.49
1
340.67
247.88
1,3
-101.64
1
12. 3FeS•.q- 1102 q- 6Na+ q- 6H•.O -- FeaO4
+ 6NaSO•
Muscovite-feldspar
equilibria
13. 3KA1Si3Os
+ 2H + = KA13Si30•o(OH)2
q-SiO9.(qtz)
q-2K+
8.18
7.72
Alunite-muscoviteequilibria
14. 2KA13(SO4)•.(OH)6
+ 6SiO2(qtz)
= 2KA13Si30•o(OH)2
+ 4H+ + 4HSO•
-27.04
-22.63
-312.34
-239.90
1,2
15. 2KA13(SO4)•.(OH)6
+ 6SiO9.(qtz)
= 2KA13Si30•o(OH)•.
+ 4H•.S+ 802
1
16. 2KA13(SO4)•.(OH)e
+ 6SiO•.•qtzl
+ 4Na+
: 2KA13Si30•o(OH)•.
+ 8H + + 4NaSO•
Alunite-kaolinite-muscovite
equilibria
17. 2KA13Si30•o(OH)2
q- 3H•O + 2H+
: 3Al•Si•Os(OH)4+ 2K+
18. 2KA13(SO4)•.(OH)e
+ 6SiO9.(qtz)
q-3H•.O+ 2H+
= 3Al•Si•Os(OH)4+ 2K+ + 4H•S•,,ql
+
-40.28
-37.87
4.46
1,3
1
-307.88
1,2
-22.58
1,2
-35.82
1,3
19. 2KA13(SO4)•.(OH)e
+ 6SiO9.(qtz)
q-3H•.O
= 3AI•Si•Os(OH)4+ 2H + + 2K+ + 4HSO4
20. 2KAla(SO4)2(OH)e
+ 6SiO9.(qtz)
q-3H•.O+ 4Na+
: 3AbSi•Os(OH)4 + 4NaSO• + 2K+ + 6H +
Alunite-pyrophyllite-muscovite
equilibria
21. 2KA13Si30•o(OH)•.
+ 6SiO9.(qtz)
q-2H+
: 3AbSi40•o(OH)•.+ 2K+
22. 2KA13(SO4)•.(OH)6
+ 12SiO•.(qtz)
q-2H+
3.63
= 3AbSi40•o(OH)•.+ 2K+ + 4H•S +
1
-236.27
1,2
-19.00
1,2
-38.32
1,3
23. 2KA13(SO4)•.(OH)e
+ 12SiO•.(qtz)
: 3AbSi40•o(OH)•.+ 2K+ + 4HSO• + 2H +
24. 2KA13(SO4)•.(OH)e
+ 12SiO•.(qtz)
q-4Na+
= 3AbSi40•o(OH)•.+ 4NaSO• + 2K+ + 6H +
Kaolinite-pyrophyllitesaturation
25.• AI•Si•Os(OH)4
+ 6H+: 2A1
+3+ 2SiO9.(qtz)
+ 5H•.O
-2.98
1,5
26.1 Al•.Si4010(OH)•.
+ 6H+ = 2A1
+• + 48iO9.(qtz)
q- 4H•.O
-10.44
27. A1OH+•'+ H + = A1+• + H•.O
28. AI(OH)• + 2H+ = A1+3+ 2H•.O
1,5
2.22
1.35
6
4.94
3.10
6
Gold saturation
29. Au + H•.S•q/+ HS- + ¬O•. = Au(HS)•
+ %H•.O
30. Au + H + + 2C1- + ¬O•. = AuCI• + «H•.O
9.70
7.55
1, 2, 7
2.23
3.23
2
References:
1 = Helgesonet al. (1978); 2 = Helgeson(1969); 3 = SmithandMartell(1976); 4 = BartonandSkinner(1979);5
= Hemingwayet al. (1982); 6 = Wolery (1986); 7 -- Shenbergerand Barnes(1989)
• ThelogK values
weredetermined
byadding
a correction
of-6.5 kJpermoleofaluminum
to thefreeenergy
valuereported
by
Helgesonet al. (1978), asrecommended
by Hemingwayet al. (1982)
1758